Page 15 - Strengths and Weaknesses of the Mathematical Model
The model has shown that for any given wall thickness, the degree of wall thickening is determined entirely by the geometric consequences of circumferential and long axis shortening, and that it is substantially greater than the 8.5% thickening of a myocyte which has contracted by 15%. Our standard ventricle of 9mm wall thickness and 5cm short axis dimension achieves a wall thickening of 28%. However this is still less than is quoted in some clinical texts. There are a few reasons why this may be:
Firstly, changes in the resting dimensions have an additive effect in the model. For the same contractility as the standard ventricle, a wall thickness of 11mm and a cavity size of 3.5cm elevate the wall thickening to 36%, but the resting dimensions would be recognised as being within normal limits.
Secondly, the model does assume that the endocardial surface is a smooth circle in systole. As you see in the images above, it is in fact a crenated surface, becoming trabecular on its inner side. The systolic image on the right shows how the crenations and some of the trabecula squeeze together to become indistinguishable. As a result the effective endocardial surface appears to be migrated inward, augmenting the measured increase in wall thickening. This problem could affect any imaging modality with enough resolution to resolve the trabecular pattern.
Thirdly, the model gives the wall thickening as a mean. In reality, the ventricle is not perfectly circular in the short axis and the repacking pattern is unlikely to be evenly distributed throughout the wall. Single line measurements of diameter may well vary from position to position.
These factors could be tested in a series of normal MRIs, by measuring the range of epicardial diameters and their percentage change, comparing the observed wall thickening with the model's predictions, and making a critical appraisal of the endocardial borders being measured.
It needs to be emphasized that the model predicts and explains the degree of thickening: it does not say how the repacking is achieved.